1. Field of the Invention
This invention relates generally to the field of switching power converters, and more particularly to digital controllers and control methods for full-bridge power converters.
2. Description of the Related Art
AC-DC power supplies which provide a high power output (greater than about 400 watts) are typically implemented using a full-bridge power converter configuration. A basic implementation of such a converter is shown in FIG. 1. A full-bridge converter employs an isolation transformer 10. Four switches (A, B, C and D)—typically transistors (shown with their respective parasitic diodes)—are used to direct a current through the isolation transformer's primary windings. There are typically two phases during which power is transferred from the primary to the secondary side of transformer 10. During a first phase (phase 1), switches A and D are closed and a current having a first polarity is conducted through the primary windings. During a second phase (phase 2), switches B and C are closed and the primary windings conduct a current having a polarity opposite the first. The switches are operated with respective pulses, which are typically generated by a pulse-width modulation (PWM) circuit (not shown).
The secondary side of the isolation transformer drives a rectification circuit 12, the output of which drives an output inductor L. The power transfer phases serve to ramp the current in inductor L up (the current is ramped down during other, non-power-transferring phases), thereby generating the converter's output voltage Vo.
During phase 1, the voltage across the primary side of transformer 10 is equal to the input voltage (+V), and a magnetic flux is building up in the transformer. During phase 2, the transformer sees an opposite voltage (−V), thus reducing the magnetic flux. For proper operation, it is important that the durations of phase 1 and phase 2 are exactly the same, and that +V=|−V| (which may be unequal due to non-ideal switches). If this is not the case, the “volt-second balance” is disturbed, resulting in a non-zero average magnetic flux in the transformer. If this imbalance becomes too large, the magnetic core of the transformer will saturate, resulting in a very high current that can easily damage the switches and the transformer.
One method of avoiding the build-up of magnetic flux is to add a capacitor in series with the primary side of transformer 10. Now, any volt-second imbalance generates a voltage across the capacitor which acts to compensate for the imbalance. However, this method requires a capacitor having a significant size and cost, and which may pose a reliability problem.
Conventionally, the switches of a full-bridge power converter are controlled using “voltage mode control” (VMC), in which output voltage Vo is compared with a reference voltage, and the error between the voltages is used to adjust the width of the pulses provided to switches A-D as needed to reduce the error. However, this control method can give rise to a volt-second imbalance and the need for a capacitor as described above.
One possible way to avoid this is to employ current mode control (CMC) instead of VMC. This method has two control loops: a voltage control loop similar to VMC, and a high-speed current control loop which measures the current in transformer 10 and provides a local feedback signal. This approach works adequately when implemented with an analog controller. However, it is difficult to implement with a digital controller.